The inherent nature of a solenoid actuated device imposes a finite delay in its response to the application of a voltage to the device. For certain types of devices, such as a fuel injector that directly injects fuel into a combustion chamber of a two-stroke internal combustion engine, commonly called a high pressure fuel injector, it becomes quite important not only to minimize this delay, but to keep the minimized delay time constant. Yet, it is equally important not to have a high current in the solenoid coil at turnoff, as again due to the inherent nature of a solenoid actuated device, this also imposes another delay when the voltage is removed. The larger the amount of energy that must be dissipated upon solenoid turnoff, the longer the delay.
The present invention relates to a switch mode circuit that responds to a pulse input signal. The pulse input signal commands actuation of the solenoid actuated device, such as the high pressure fuel injector and the circuit creates a particular shaped voltage waveform across the solenoid coil. This voltage waveform controls a current through the solenoid coil that is effective to actuate the device with improved quickness. Once actuated, the circuit causes the amount of current to drop, at a controlled rate, to a hold level that is sufficiently high to assure that the solenoid remains actuated but at the same time is sufficiently low to assure that the energy will be dissipated quickly when the pulse signal is removed.
The invention is embodied in an electronic control power circuit system which comprises a low-current signal processing portion and a high power switching portion that controls the current through the solenoid coil in accordance with the control provided by the signal processing portion. While the preferred embodiment of the invention comprises its signal processing portion constructed from discrete electronic circuit components, it should be understood that such signal processing may be performed in an equivalent way by the use of a microprocessor that executes suitable algorithms for performing the equivalent functions performed by the disclosed signal processing portion.